The composite aircraft skin structure shown in FIG. 1 comprises a panel 1 with an inner mould line surface (IML) formed with a ramp 2, and an outer mould line surface (OML) 1′ which forms an outer aerodynamic surface of an aircraft. A pair of stringers 3, 4 are attached to the IML, and run spanwise along the length of the panel, stiffening the panel against bending in the spanwise direction.
The skin is joined to a rib via three rib feet 5-7, separated by a pair of “mouse hole” slots 3′,4′ which provide clearance for the stringer blades. Each rib foot 5-7 has a respective flange 5′-7′ extending at right angles to the web of the rib foot. The skin is joined to the rib by passing fasteners 9 through the stringer feet, the panel and the rib foot flanges.
A problem with the structure shown in FIG. 1 is that the ramp 2 must be relatively shallow (typically with a gradient no greater than 1:20 relative to the OML) to avoid load eccentricities. Also, it is preferable for the IML to be parallel to the OML under the stringer feet (because this makes it easier to mount the rib feet onto the stringer feet) and as a result it is preferred for the ramp not to extend under the stringer feet. Consequently it is difficult to achieve a significant variation in panel thickness between the two stringers. Such a large variation in thickness may be desirable for example in the region of an engine or pylon mount, where a thicker skin is required.